In the prior-art dryer-section concepts based on single-wire draw, in view of prevention of curling of paper, it is necessary to invert at least one group, in which case the paper is also dried from the other side. It has been noticed that, in the event of a web break, such so-called inverted groups with single-wire draw are difficult to clean, and the fibre strings produced may break wires and blow boxes and produce unnecessary losses in production.
The system of the present sort can be abandoned by using a drying geometry in which the dryer section consists of drying cylinders and of impingement units fitted on said cylinders, out of which impingement units hot air/steam is blown through the wire onto the paper. By means of such an impingement unit, a considerable increase in the evaporation capacity is achieved. In such a case, the evaporation takes place increasingly from the wire side on the cylinder. Owing to the increased evaporation capacity, the dryer section can be made shorter, thus obtaining economies in the cost of construction of the hall. Owing to the increased evaporation capacity, the concept of the invention can also be applied to modernizations, in which the available space is often quite limited.
From the application JP 222,691/1993, a dryer section is known in which there is an impingement hood placed on the top of all of the upper cylinders. In the present invention of ours, the impingement hoods are preferably placed in connection with the cylinders only at which the hoods provide a considerable effect either on the control of curling or on an increased drying capacity.
When the impingement is carried out through the wire, the paper is protected between the wire and the cylinder and it has no possibility to form so-called agglomerations, which might collide with the impingement device itself and damage it.
An increase in the evaporation on the cylinder normally results in a lower average temperature of the web and thereby slightly reduces the evaporation taking place in the area of free draw, but, on the other hand, the delivery of heat by the cylinder is increased, which increases the overall evaporation.
The impingement unit can also be divided into blocks in the direction of width of the machine, in which case, by means of the unit, it is possible to act upon the evenness of the moisture profile in an attempt to improve the uniformity of the final product in the cross direction of the web.